Video color images are typically formed using an array of small groups of red, green, and blue pixels. When the relative contributions of these three colors in an RGB pixel group are controlled, these three colors combine to create all colors in the video image. Projection display systems typically operate by illuminating one or more light modulators with very bright red, green, and blue light sources. The light source may be a very bright white light whose light is filtered to create red, green, and blue components. Such a white light source generates much heat and is inefficient since much of the light generated is other than red, green, and blue and is thus wasted. A more efficient light source consists of red, green, and blue LEDs, since no filtering is required and all the light generated is used to create the gamut of colors in the displayed image. The present application is directed to projection systems using LED light sources.
The light modulators may be small liquid crystal panels (called micro-displays) for each primary color. The red images, the green images, and the blue images are then combined by optics and projected on a screen. The projection may be a front projection or a rear projection.
Some other types of light modulators are micro-electro-mechanical system (MEMS) devices, such as the digital light processor (DLP™) made by Texas Instruments, where an array of micro-mirrors rapidly reflect red, green, and blue light components onto a screen. Each mirror corresponds to a pixel in the display. The angles of the mirrors determine whether the pixel is on or off, and the duty cycle determines the RGB components at each pixel location.
For large screen projection systems, the light must be very bright. To achieve such high brightness, multiple high power LEDs of each color may be used. There may be a small array of LEDs for each primary color to obtain the desired brightness.
Due to the relative efficiencies of red, green, and blue LEDs, combined with the human eye's different sensitivities to red, green, and blue light, the power used to generate the required red light component for a certain white point is much greater than the power used to create the blue light component for that white point. Since red LEDs become less efficient at higher temperatures, this relative inefficiency is exacerbated when the red LED is a high power LED that generates heat. To a lesser extent, the power used to generate the required green light component for the white point is greater than the power used to create the blue light component for that white point. However, the relative efficiencies of the red and green LEDs vary with manufacturer and, hence, green LEDs may be less efficient than red LEDs in a display in some cases.
This is a result of the following characteristics of light and LEDs. A measure of the perceived brightness to the human eye is in units called lumens. The ratio of lumens/watt is called efficacy. The human eye is much more sensitive to green light than to blue and red light. For standard red, green, and blue LEDs, assume red LEDs output around 40 lumens/watt (electrical), green LEDs output around 100 lumens/watt (electrical), and blue LEDs output around 20 lumens/watt (electrical). More efficient LEDs have a higher efficacy but the efficacy relationships between the colors generally remain the same, assuming the red, green, and blue LEDs are of the same quality. To create white light (e.g., 6500-9000 K), the relative lumen contribution is about 25% red, 70% green, and 5% blue. Blue LEDs convert electrons into emitted photons at a percentage (about 40%) that is more than double the percentage of red and green LEDs. In view of the above characteristics, to create white light from LEDs, much more power is needed for generating red light than for generating blue light. Additionally, to create white light, more power is needed for generating green light than for generating blue light.
What is needed is a technique to increase the efficiency of an LED light source in a projection display.